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can lay out engineer change the width of a resistor?

VEERA PRAMOD

Hi all...

Can layout engineer change the width of a resistror by keeing the resistance constantly as in schematic? And what is the minimum length of  a resistor ?is it depends on width?

 

Thnk you.

 Veera Pramodu Kotrika.

  • Hi Veera,

    Did you really just ask this question?  I'm surprised that such a basic question is being asked, did Google not provide an answer?

    Anyway, to answer your question, the resistance of a wire (and a resistor made of resistive material, therefore) is directly proportional to the length of the wire and inversely proportional to the cross-sectional area.  There's a formula for this:

    R = p L
           A

    Where R is the resistance, "p" is rho, a resistivity constant, and L and A are length and area respectively. When laying out a resistor, you do not have control over the thickness of the resistive material (usually, but there might be some process out there...) but you do have control over the width - obviously Area = thickness X width so the area is directly proportional to the width.

    The minimum length will depend on the process limits, and these are most likely to be set and enforced by the Process Design Kit, likewise the width will have a minimum as defined by the manufacturing process.

    Hopefully this answers your question, if not, please read the hint in my first line of response.

    Regards,

    Lawrence.

  • Hi Lawrence,

    My designer has provided a resistor w/l as 1.5u/15.My doubt is may i change it to 2u/20 or 1u/10u width & length respectively with out the designer permission.If it is a simple question to you i'm sorry.Really i don't know may i do like this or not.

    Thank you.

    Veera Pramodu Kotrika.
  • Lawrence,

    I suspect the question is more to do with whether the specific PDK being used allows the width of the resistor to be changed, keeping the resistance constant.

    However, that would be entirely dependent on the design kit, and since Veera didn't say which it was, nobody could realistically answer.

    That said, one should be wary of changing the width because the designer will have simulated the design with a particular width - this may be important for matching reasons - and it may also have some more subtle effects depending on how it is modelled.

    Regards,

    Andrew
  • Veera,

    See my response I just sent. In general the answer would be that you could do this, but it depends on the PDK you're using, and you didn't say what that is.

    Regards,

    Andrew

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